Polyurethane foams are generally prepared by the reaction of an active hydrogen-containing compound (i.e., a polyol) and a polyisocyanate, in the presence of a blowing agent such as water, and usually a reaction catalyst and foam stabilizer. The cellular polymer structure of polyurethane foam has a skeletal framework of relatively heavy strands forming an outline for the cell structure. The skeletal framework strands are connected by very thin membranes, often called windows, which form the cell walls. In open-celled foams, some of the windows are open or torn in each cell, thus forming an interconnecting network open to fluid flow (liquid or gas). However, conventional polyurethane foams are not sufficiently porous or open-celled to allow significant fluid flow through the foam structure.
Reticulation relates to methods for removing or breaking the cell windows of polyurethane foams. Mechanical, chemical and thermal methods for reticulating foams are known. As one example, a foam may be reticulated by melting the windows with a high temperature flame front or explosion, which still leaves the strand network intact. Alternatively, the cell windows may be etched away using the hydrolyzing action of water in the presence of an alkali metal hydroxide. See U.S. Pat. Nos. 3,125,542; 3,405,217; 3,423,338; 3,425,890 and 4,670,477 for descriptions of various reticulating methods for polyurethane foams.
Household cleaning sponges and mop heads most commonly are formed from cellulose. Paper pulp is the primary ingredient for cellulose sponges. The pulp is reacted with carbon disulfide to form a soluble cellulose xanthate compound. This compound is dissolved into a honey-like liquid viscose and mixed with reinforcing fibers to add strength to the pulp mixture. The cellulose is formed with a double cell structure to replicate natural sea sponges. Sodium sulfate crystals are added to the pulp, and this mixture is heated in a mold to melt the crystals. Heating regenerates the mix to pure cellulose and leaves the signature sponge holes where the crystals have melted away. Bleaching chemicals and humectants maintain the moisture level and color purity of the cellulose sponge. While the cellulose has good water absorption and wicking, it has lower wet integrity than other materials. Moreover, upon drying, the cellulose becomes hard and brittle such that it must be pre-wet before using for wiping.
Open celled ester and ether polyurethane foams have greater softness and flexibility than cellulose, and retain flexibility upon drying without humectants. As compared to cellulose, foams have greater wet strength, better wet integrity and exhibit less swelling when wet. Foams also can be foamed to have a double cell structure to more resemble natural sea sponges. Generally, polyurethane foams can be produced more cheaply than cellulose. However, polyurethane foams are hydrophobic, lacking good liquid absorption and wicking characteristics, which makes them less suitable for household sponges and mop heads. Even after the polyurethane foams are post-treated with surfactants in an attempt to improve water absorption and wicking, they still do not match the performance of cellulose for these properties.
U.S. Pat. No. 6,756,416 discloses chemically reticulated ester polyurethane foams that have liquid absorption and wicking characteristics comparable, if not superior to, cellulose sponges. After the ester polyurethane foams are chemically treated in a caustic solution, they have water absorption rates of at least 20 pounds of water per square foot per minute. Such foams also have greater water holding capacity and wet strength than cellulose.
Often in designing sponges for household consumer and industrial applications, multiple material layers are combined together to offer additional properties and alter sponge appearance. For example, a hydrophilic ester polyurethane layer may be combined with a layer of nonwoven or another polymeric layer, such as a melamine or a polyethylene foam. When foams and other layers are bonded together, some manufacturers prefer that the bond be transparent to the consumer. In other words, consumers prefer sponges that do not have a stiff glue line between layers.
One foam joining method without glue or adhesive is flame lamination. A surface of a foam layer is heated to a temperature sufficient to locally melt and repolymerize the foam. The foam layer is then joined to a second layer while the surface is polymerized. Upon cooling, the melted surface hardens to form the bond between the foam layer and the second layer. Non-hydrophilic ester polyurethane foams are commonly recognized to give a stronger flame lamination bond than ether polyurethane foams. See U.S. Pat. Nos. 5,900,087 and 5,891,928. On the other hand, flame lamination using hydrophilic ester polyurethanes has seldom been practiced, and its limitations rarely studied.
When flame-laminated, non-hydrophilic ester polyurethane foams typically have a bond strength of over 10 oz/inch [109 N/m], as measured by tear strength test method ASTM D3574. By contrast, the hydrophilic ester foams typically have lower bond strengths, such as in the range of 4 to 9 oz./inch [43.8 to 98.5 N/m]. Although this lower bond strength could be satisfactory for some applications if it were uniform across a substrate and predictable, the bond strength appears to vary with the relative humidity of the process area. As the relative humidity increases, the bond strength drops to unacceptably low levels, particularly at the outer edges of the surface to be laminated.
Industry thus seeks ways to improve flame lamination for hydrophilic ester polyurethane foams.
One approach to improving flame lamination bonding uses a sacrificial non-hydrophilic ester foam layer between layers sought to be joined. This approach has been used when bonding ether polyurethane foam layers together because some ether polyurethane foams are known to form only very weak flame lamination bonds by themselves. A sacrificial foam layer typically is not required when bonding ester polyurethane foams because non-hydrophilic ester foams in general are considered flame laminable. Nevertheless, this has not been true for hydrophilic ester polyurethane foams. In view of the lower bond or tear strength exhibited and the adverse effects of relative humidity, improvements for flame lamination bonding of hydrophilic ester polyurethane foams continue to be sought.